1. Field of the Invention
The present invention relates generally to selective electroplating techniques and products produced thereby, and more particularly to novel methods for the selective electroplating of precious metals such as gold onto various types of base materials.
2. Discussion of the Prior Art
Electroplating, that is the electrodeposition of an adherent metallic coating onto a base material, is a well known and highly useful technique. Coatings produced by electrodeposition methods may perform a utilitarian function, such as preventing corrosion of the base metal, e.g., plating with zinc (electrogalvanizing) or with tin, or they may perform a decorative function, e.g., plating with gold or silver to produce fine jewelry and the like, or they may perform both functions, e.g., brass plating of hardware parts. Another important utilitarian application of electroplating, and one particularly pertinent to the present invention, relates to the electrodeposition of metallic coatings onto electronic components interfaces to facilitate the bonding together of the components by such techniques as ultrasonic and thermo-compression bonding as well as by more conventional bonding techniques, such as soldering, brazing, and the like. For example, in the production of electronic parts such as semiconductor devices, gold plating is widely used on the mating interfaces of the various component parts of the devices to provide low electrical contact resistance and to insure proper bonding during final assembly of the semi-conductor package. Although a wide variety of precious metal coatings are used in the electronics field to facilitate bonding and enhance reliability, gold plating is perhaps the most common. Particularly in the production of semiconductor devices, gold provides many highly desirable properties. It is corrosion resistant, has good electrical conductivity, readily accepts solder, can be eutectically bonded to silicon chips, can be ultrasonically bonded, can be thermo-compression bonded, and does not oxidize at elevated temperatures.
Because of the numerous advantages provided by precious metal coatings, very significant quantities of metals such as gold, silver, rhodium, palladium and platinum are presently being used by both domestic and foreign industries. However, because of the extremely high cost of these metals, there has been a continued on-going effort to find ways to reduce or avoid their use. Avenues of investigation have included extensive evaluation of materials such as aluminum and various non-precious metal alloys as substitutes for the gold or other precious metals. Also, considerable effort has been directed toward the development of methods and techniques for precision selective electroplating of various types of precious metals.
Efforts directed toward the substitution of less expensive metals and metal alloys for the precious metals has found limited application. Frequently, it has been found that savings achieved by reductions in the amount of the precious metal used are offset by the loss of performance and reliability of the end product or by reduced production rates and production yields directly attributable to the use of the substitute materials.
Techniques developed for the selective electroplating of the precious metals have shown somewhat more promise. However, a great many of the prior art techniques which have been suggested for selective electroplating, although successful in reducing the quantities of precious metal used, require costly specialized equipment and result in reduced production rates.
Before undertaking a discussion of the numerous advantages of the method of selective electroplating of the present invention over prior art techniques, a brief review of prior art methods is perhaps in order. In the main, the prior art techniques for selective electroplating are of two general types. Those in which the areas of the part which are not to be plated are isolated or masked off by mechanical means such as by clamping or otherwise affixing reusable templates or the like to the articles to be plated; and those which involve the use of some type of discardable plastic or non-conductive masking material which is sprayed on, bonded or otherwise affixed to the area of the part which are not to be plated.
The significant drawback of the first mentioned class of prior art methods is the mechanical complexity and concomitant high cost of the fixturing required to support large volume production. The significant drawback of the second class of methods is the difficulty and resulting high cost of first applying and later removing masking material from the part. Such difficulties are particularly pronounced when the components to be selectively plated are very small and of complex, irregular configuration such as semiconductor devices, miniaturized printed circuit boards, and the like.
A major drawback of both classes of the aforementioned prior art techniques is that the masked off components often cannot be electroplated by conventional low-cost barrel plating methods. It is a fundamental requirement of all barrel plating processes that the components to be plated be electrically conductive, i.e., be able to conduct electrical current from part to part within the mass of parts being plated within the barrel. The use of masking templates or coatings formed of a plastic base material such as those used in the prior art selective electroplating techniques make barrel plating impractical.
Because of these and other drawbackss of prior art selective electroplating methods, many industrial concerns, particularly those involved in the manufacture of electronic components, are often forced to apply the precious metal coating over the entire component rather than trying to confine the coating only to critical areas.
When it is realized that in the production of many types of electrical components by present techniques the area of the parts which are actually being plated with precious metal substantially exceeds that which is required to be plated for functional and performance reasons, the significant commercial value of the present invention becomes readily apparent.